There may be a demand in the future for optical transmission networks that use OTN (Optical Transport Network) to accommodate interfaces with low jitter, such as a CPRI (Common Public Radio Interface: wired interface between wireless base station and slave station) and a 3G SDI (3rd Generation Serial Digital Interface: 3rd generation analog video digital distribution interface). In protocols, such as PDH (Plesiochronous Digital Hierarchy), SDH (Synchronous Digital Hierarchy), and OTN, client signals are accommodated in a server signal using a different clock source by using an asynchronous accommodating method, such as stuffing synchronization, pointer synchronization, and GMP (Generic Mapping Procedure).
However, the problem with the asynchronous accommodating method is that low frequency jitter known as WTJ (Waiting Time Jitter) is generated due to the frequency difference between client signals and a server signal. With the asynchronous accommodating method, client signals are written in the payload portion of the server signal on the transmission side. At this point in time, because the client signal frequency (fClient) and the data rate of the server signal payload (fpayload) are asynchronous, they do not match each other. This causes a loss or insufficiency of data.
In order to prevent the loss or insufficiency of data, a stuff area is prepared in the server signal for making adjustments. The synchronizer on the transmission side compares the write address with the read address in the elastic store memory. When fClient is larger than fpayload, the synchronizer performs negative stuffing to extend the payload area. When fClient is smaller than fpayload, the synchronizer performs positive stuffing to reduce the payload. Thus, the amount of data in the client signals and that in the server signal are made equal to each other. On the reception side, data is extracted from the payload and is written in the elastic store memory, and a write counter is used to extract the original client signal timing, which is caused to pass through a PLL (Phase Locked Loop) in order to extract the smoothed original client signal frequency.
When the frequency difference between fClient and fpayload is small on the synchronizer side, the stuffing operation is performed with a low cycle. The bandwidth of a PLL is a few tens of Hz or higher; therefore, WTJ, which is low-frequency jitter components generated by performing a stuffing operation, cannot be removed.
An example of a measure against. WTJ is a method in which the synchronizer randomly changes the stuffing operation threshold in the elastic store memory. With this method, jitter components are intentionally changed to high-frequency components so that they can be removed in the subsequent PLL, thereby reducing the total number of jitter components. If the stuff generation frequency is low, low-frequency jitter is generated. Thus, the amount of stuff each time is preferably low. A technology is disclosed, for example, in Patent Literature 1 below, in which the de-stuffing timing is made random in order to reduce the amount of jitter.